The present invention relates to a magnetic field sensor which comprises a Hall element and an amplifier for amplifying the output voltage of the Hall element and which detects the magnetic field strength in the installed location so as to output a signal in accordance with the detected magnetic field strength.
A typical magnetic field sensor is a bipolar IC or a CMOS IC which include a Hall element for outputting an output voltage proportional to the magnetic field strength and an amplifier for amplifying an output voltage of the Hall element as well as a comparator for inputting the output voltage of the amplifier to be compared with a reference potential and for outputting the comparison result. Such a magnetic field sensor outputs an output signal of two values (0 or 1) showing whether the magnetic field strength of the location where the magnetic field sensor is installed is larger or smaller than a constant reference.
Another magnetic field sensor comprises a Hall element for outputting the output voltage proportional to the magnetic field strength and an amplifier for amplifying the output voltage of the Hall element and outputs the output signal of that amplifier as an analog signal, without change.
One of the major factors of dispersion in characteristics among the products of the magnetic field sensor is the dispersion of the offset signal component included in the output voltage of the Hall element. This occurs due to the stress, or the like, which is received by the Hall element body from the package. Another one is an offset signal component which exists at the input terminal of the amplifier (in general, a differential amplifier).
U.S. Pat. No. 4,037,150 discloses a technology which makes the influence of the offset signal component of the Hall element be small. A magnetic field sensor according to the invention described in U.S. Pat. No. 4,037,150 has a Hall element in a plate form with four terminals and the form of a Hall element is geometrically equal, as is that of the Hall element 1 described in FIGS. 5 and 6.
“Geometrically equal forms” means that the form under the condition of FIG. 5 and the form under the condition where the Hall element of FIG. 5 is rotated by 90 degrees (it is rotated so that A-A′ agrees with B-B′ in FIG. 5) are the same, as is the Hall element 1 described in FIG. 5.
A description is made in reference to FIG. 5. The Hall element has two pairs of terminals A-A′ and B-B′ in the diagonal direction. In the first phase (first timing) a power source voltage is applied across the terminals A-A′ and the output voltage across the terminals B-B′ is detected so as to be stored in memory. Next, a power source voltage is applied across the terminals B-B′ at the second phase (second timing) and the output voltage across the terminals A-A′ is detected so as to be stored in memory. The switching of these actions is implemented by the switch circuit 24.
Here, a circuit for applying a power source voltage to the Hall element is not shown in every figure.
The timing chart for the first and the second phases is described in FIG. 7. A sum is gained between an output signal of the first phase and an output signal of the second phase and, then, an effective signal component of an output signal of the Hall element is added in the same phase so as to be doubled while an offset signal component of an output signal of the Hall element is added in the negative phase so as to be mutually canceled. In this manner, the influence given to the output signal by the offset signal component of the Hall element is suppressed.
Next, the configuration of a conventional magnetic field sensor which compensates the offset signal component due to the input offset of the amplifier is described in reference to FIGS. 5 and 6.
FIG. 5 shows a configuration of a magnetic field sensor according to the first prior art as disclosed in the Japanese unexamined patent publication H8(1996)-201491. In FIG. 5, a Hall element is denoted as 1, a switch circuit is denoted as 24, capacitors which are memory elements are denoted as 4 and 6, switches are denoted as 5 and 8, voltage-current conversion amplifiers, each of which has high input and output impedance and converts a input voltage into a current so as to be outputted, are denoted as 10 and 11, and a resistance is denoted as 12.
In the first phase, the first phase signal (a) which has a pulse is given to the switch 5 while in the second phase, the second phase signal (b) which has a pulse is given to the switch 8. In addition, the first and the second phase signals are given to the switch circuit 24.
The relationship between the first phase and the second phase in the first prior art is shown in FIG. 7.
The operation in the first phase is described.
In the first phase the switch 5 is closed while the switch 8 is open. At this time, a power source voltage is applied across the terminals A-A′ of the Hall element 1 so that the output voltage across the terminals B-B′ is outputted through the switch circuit 24. The output voltage of that Hall element 1 is inputted to the voltage-current conversion amplifier 10.
The voltage-current conversion amplifier 10 outputs a current which is proportional to the output voltage of the Hall element 1. The output current IOUT of the voltage-current conversion amplifier 10 is represented as in the following equation.
 IOUT=α(Vh+Voff10)  (1)
Voff10 is an input offset voltage of the voltage-current conversion amplifier 10 and Vh is an output voltage of the Hall element (input voltage of the voltage-current conversion amplifier 10). α is a conversion coefficient (proportional constant) from voltage to current.
The resistance value of a Hall element has a great dispersion among products. In general, when the resistance value of a Hall element is small, the output voltage of the Hall element becomes large and when the resistance value of the Hall element is large, the output voltage of the Hall element becomes small.
This current flows into the capacitors 4 and 6 via the switch 5. A voltage-current conversion amplifier 11 which has the same functions as the amplifier 10 generates a current which is proportional to a differential voltage between the charging voltage of the capacitor 4 and the charging voltage of the capacitor 6 and which is in the opposite direction to the direction of a current of the voltage-current conversion amplifier 10. Charging current to the capacitors 4 and 6 stops when the sum of the respective output currents of the voltage-current conversion amplifiers 10 and 11 becomes zero. At this time, since the directions of the output currents of the respective voltage-current conversion amplifiers 10 and 11 are opposite to each other, the absolute values of the respective output currents of the voltage-current conversion amplifiers 10 and 11 agree. Accordingly, the output current IOUT2 of the voltage-current conversion amplifier 11 can be represented in the following equation.IOUT2=−α(Vh+Voff10)  (2)
Next, the operation in the second phase is described.
In the second phase, the switch 5 is open and the switch 8 is closed. At this time, since the charging and discharging currents for the capacitors 4 and 6 do not flow, the capacitors 4 and 6 maintain the charges (accordingly, voltage) stored in the first phase. Accordingly, the voltage-current conversion amplifier 11 makes the current of the same value as of the current in the first phase keep flowing. The output current IOUT2 of the voltage-current conversion amplifier 11 is represented in the equation (2).
At this time, a power source voltage is applied across the terminals B-B′ of the Hall element 1 so that the output voltage across the terminals A-A′ is outputted through the switch circuit 24. The output voltage of that Hall element 1 is inputted into the voltage-current conversion amplifier 10. The output signal of that Hall element which has been inputted into the voltage-current conversion amplifier 10 is substantially in the opposite direction to that at the time of the first phase. Accordingly, at this time, the output current of the voltage-current conversion amplifier 10 becomes of the same amount and of the same polarity as of the output current of the voltage-current conversion amplifier 11.
The output current IOUT1 of the voltage-current conversion amplifier 10 in the second phase can be represented in the following equation.IOUT1=α(−Vh+Voff10)  (3)
The sum current of the output currents of the voltage-current conversion amplifiers 10 and 11 flows into the resistance 12 via the switch 8.
Therefore, the current I which flows into the resistance 12 is gained by adding the equation (2) and the equation (3) as:I=IOUT1+IOUT2=−2αVh  (4)which shows that the input offset voltage Voff10 is canceled.
FIG. 6 shows the second configuration example of a conventional magnetic field sensor. In FIG. 6, a Hall element is denoted as 1, a switch circuit is denoted as 24, a voltage amplifier is denoted as 25, capacitors which are memory elements are denoted as 4 and 6, and switches are denoted as 5, 8 and 9. The capacitance values of the capacitors 4 and 6 are equal.
The relationship among the first phase, the second phase and the third phase according to the second prior art is shown in FIG. 8.
The operation in the first phase is described.
In the first phase, the switch 5 is closed while the switches 8 and 9 are open.
At this time, power source voltage is applied across the terminals A-A′ of the Hall element 1 so that the output voltage across the terminals B-B′ is outputted through the switch circuit 24. The output voltage of that Hall element 1 is inputted into the voltage amplifier 25.
The voltage amplifier 25 outputs the voltage proportional to the output voltage of the Hall element 1. The output voltage V1 of the voltage amplifier 25 in the first phase can be represented in the following equation.V1=β(Vh+Voff25)  (5)
Voff25 is an input offset voltage of the voltage amplifier 25 and Vh is an output voltage of the Hall element (input voltage of the voltage amplifier 25). β is a voltage amplification factor of the voltage amplifier 25.
The capacitor 4 is charged to the output voltage V1 of the voltage amplifier 25 via the switch 5.
Next, the operation in the second phase is described.
In the second phase, the switch 8 is closed while the switches 5 and 9 are open.
A power source voltage is applied across the terminals B-B′ of the Hall element 1 so that the output voltage across the terminals A-A′ is outputted through the switch circuit 24. The output voltage of that Hall element 1 is inputted into the voltage amplifier 25. An output signal of that Hall element which is inputted into the input terminal of the voltage amplifier 25 becomes substantially of the opposite direction to that at the time of the first phase. Accordingly, at this time, the output voltage V2 of the voltage amplifier 25 can be represented in the following equation.V2=β(−Vh+Voff25)  (7)
The capacitor 6 is charged to the output voltage V2 of the voltage amplifier 25 via the switch 8.
Finally, the operation in the third phase is described.
In the third phase, the switch 9 is closed while switches 5 and 8 are open.
Both terminals of the capacitor 4 are made to cross each other via the switch 9 and are connected in parallel with both terminals of the capacitor 6. As a result, the average value of the voltage −V1, across the terminals of the capacitor 4, and the voltage V2, across the terminals of the capacitor 6, is outputted to the output terminal. Since the capacitance values of the capacitors 4 and 6 are the same, that output voltage V is represented in the following equation.V=(−V1+V2)/2=−βVh  (8)
Here, it is seen that the input offset voltage Voff25 of the voltage amplifier 25 is canceled.
The voltage amplifier 25 of the magnetic field sensor, which utilizes a Hall element, outputs the first output signal which is a signal gained by amplifying the output signal across the two mutually facing terminals of the Hall element with four terminals in the first phase. The voltage amplifier 25 outputs the second output signal which is a signal gained by amplifying the output signal across the other two mutually facing terminals of the Hall element with four terminals in the second phase. This second output signal is substantially a signal gained by inverting the first output signal. In this manner, the voltage amplifier of a magnetic field sensor which utilizes a Hall element cancels the input offset voltage Voff25 of the voltage amplifier 25 by outputting signals in the first phase and in the second phase which are in a substantially inverted relationship.
Since the output voltage of the Hall element is outputted as a differential voltage between the two terminals of the Hall element, conventionally the differential voltage of the Hall element is inputted into a differential amplifier so that the differential amplifier outputs a non-inverted (plus) output signal and an inverted (minus) output signal.
Therefore, an amplifier of a conventional magnetic field sensor is a double output-type amplifier which has a non-inverted output terminal and an inverted output terminal as shown in FIG. 5 or 6.
When the double output-type amplifier is used, however, the output part has a large number of component elements and a large chip area is occupied.
In a conventional configuration, there is the defect that the circuit scale for canceling the input offset voltage is large.
In addition, a magnetic field sensor has been used in products which are battery operated, such as cellular phones, in recent years and, therefore, the reduction of the consumption current of the magnetic field sensor is becoming an important technical problem. As for the means used for the reduction of the consumption current, it is general to adopt an intermittent operation which makes the consumption current be zero during a constant time by using a counter, or the like.
However, there is a constraint in the time wherein the sensor operation can be stopped depending on the sets using the magnetic field sensor and, therefore, it becomes a problem of in how many steps one sensing operation can be implemented. More concretely, in the first prior art, the magnetic field strength can be measured in the two steps of the first and the second phases. In the second prior art the magnetic field strength can be measured in the three steps of the first to the third phases.
The present invention is intended to solve the above described conventional problem and has the purpose of providing a magnetic field sensor which reduces the dispersion of the output voltage for detecting the magnetic field strength and which consumes a small amount of power and is inexpensive.